1. Field of the Invention
The present invention relates to the connection of optical fibers having different optical properties. More particularly, the present invention relates to an optical fiber which can be used as a bridge to connect two other optical fibers. While the invention may be suitable for a wide range of connectivity applications in telecommunications systems, a fiber disclosed herein is especially well suited for connecting step index single mode fibers to dispersion compensation fibers having complex refractive index profiles, and will be particularly described in that regard.
2. Technical Background
Dispersion compensation techniques may be successfully used in telecommunications systems or links. Total dispersion, or chromatic dispersion, may be compensated by an appropriately designed waveguide fiber contained in a dispersion compensating module that can be inserted into the link at an access point such as an end of the link. The compensating waveguide fiber can be designed to allow operation in, for example, the 1550 nm operating wavelength window of a link that was originally designed for the 1310 nm operating window. A disadvantage of compensating with a module is that attenuation and nonlinear penalties are added to the link without increasing the useful link length. Furthermore, refractive index profile designs for such dispersion compensation are typically more complex, more difficult to manufacture, and may have higher attenuation than the so-called transmissive fibers making up the link. In some designs, the compensation parameters may be achieved at the expense of the effective area of the optical fiber, but a lower effective area compensating waveguide fiber is more susceptible to non-linear effects.
Another approach to dispersion compensation is providing both positive and negative dispersion fibers in the cables of the link, wherein both the positive and negative dispersion fibers are transmissive fibers, that is, the compensative fiber or fibers are not wound around bobbins inside a module, but rather add to the useful link length. Each cable can contain both positive and negative total dispersion waveguide fibers, or the link can be formed using cables having only positive dispersion in conjunction with cables having only negative dispersion. However, as in the dispersion compensating module approach, a relatively high attenuation and low effective area of the negative dispersion fiber can be problematic in this approach as well. Furthermore, cable inventory must typically be managed carefully, because replacing or repairing a cable may involve tracking of one or more other variables, such as the sign of the dispersion of fibers in the cable. In certain profile designs, a mismatch of mode fields between the positive and negative total dispersion fibers may exist, resulting in relatively large or excessive splice or connecting losses.
In the latter approach, uncoiled lengths of optical fibers such as single mode fibers and compensative fibers such as dispersion compensating fibers can be effectively used together in optical systems, although to achieve desirable or necessary system performance, such fibers typically need to be connected or spliced by other optical fibers or optical devices without exhibiting excessive connection losses or xe2x80x9csplice lossesxe2x80x9d. Typically, the positive dispersion single mode transmissive fibers have mode field diameters that differ in size and in other aspects from the mode field diameters of the negative dispersion compensative fibers to which the single mode fibers will be connected. The direct connection of fibers having such mismatched mode field diameters generally results in excessive splice loss.
A number of techniques have been developed over the years to limit the adverse effect of splice loss resulting from mode field diameter mismatch. Heretofore, physical tapering, in-line optical devices, and thermally diffused expanded core (xe2x80x9cTECxe2x80x9d) methods have been employed in an attempt to adequately match the mode fields of fibers and other devices having different mode field diameters. Physical tapering includes both down-tapering and up-tapering. TEC methods include those methods used to expand the mode field diameter via diffusion.
In the down-tapering method, the optical fiber is first fusion-spliced by conventional methods, and the spliced portion of the fiber is thereafter heated so that it can be stretched by pulling. In this way, the softened spliced portion develops a tapered shape. The reduced core misalignment due to the tapered shape and the spreading of the mode field diameter in the smaller core diameter fiber typically result in lower splice loss when compared to the original non-tapered splice. However, the tapers fabricated by this method are sensitive to physical perturbations or external refractive-index change because the mode field is no longer tightly bound to the core. In addition, the outer diameter of the tapered fiber changes during the drawing process, thus special fiber plugs are typically required for connections.
An up-taper is fabricated at the stage of drawing a preform and results in an enlarged core. The enlargement of the core results in an expanded mode field diameter. This method is typically applicable for mechanical splicing, bonded splicing, or connectors between an erbium-doped fiber (xe2x80x9cEDFxe2x80x9d) and a single mode fiber. However, this method also requires special plugs for the connectors, and additionally requires a special preform.
The thermally diffused expanded core method entails heating a fiber to cause dopant diffusion to expand the mode field diameter. Generally, the fusion connection of two fibers with different mode field diameters with this method is made by continuously or adiabatically varying the core diameters of one or both fibers so that the mode field diameters match at their boundaries. During the process of dopant diffusion, the core diameter locally becomes large, and the relative refractive index difference locally becomes small, resulting in a tapered core and a tapered mode field diameter along the length of the fiber. Accordingly, the thermally diffused expanded core method can be an effective method for locally expanding the fiber mode field diameter. However, implementation of the thermally diffused expanded core technique generally involves either heat-treating a smaller mode field diameter fiber in a furnace or a gas burner, then fusion-connecting the expanded fiber with the larger mode field diameter fiber, or first fusion-connecting the two fibers, then applying additional heat to diffuse the fused region.
When furnaces or microburners are employed to provide the heat for the diffusion, the process typically takes several hours to complete, due to the temperature limits of most furnaces, and may require the application of a carbon coating once the primary coating has been stripped from the fiber to reduce the heat exposure time required to properly diffuse the dopant. However, application of a carbon coating is expensive and time consuming. Furthermore, long periods of exposure to a gas flame tends to make a fiber brittle even if temperatures within the furnace are not extreme. Thus, for example, open-ended furnaces having a maximum temperature of approximately 1300xc2x0 are employed to treat the fiber in such a process. Using such an open-ended furnace generally requires exposing a fiber having a 1% maximum relative refractive index difference for more than ten (10) hours, for example. Typically, the low temperature gradient in an open-ended furnace allows the fiber core to expand slowly. Typically, however, the relatively long heat-treated section of the fiber has relatively low mechanical strength and requires extra protection and packaging. Moreover, the first implementation of the thermally diffused expanded core technique is generally either not available or not practical in the field, where many of the fiber splices must be made, because of the large size of the required furnace and microburner systems.
The second implementation method of the thermally diffused expanded core technique typically works well when the diffusion coefficient of the core dopant in the smaller mode field diameter fiber is much greater than that of the larger mode field diameter fiber. A small mode field diameter fiber doped with erbium is a typical example. However, for fibers with a relatively large maximum relative refractive index difference and for single-mode fibers, both of which may be doped with slowly diffusing germanium, the core discontinuity cannot be completely eliminated using this method. Furthermore, when a splice is achieved with an arc fusion discharge, the resulting splice loss is typically around 0.3 dB, which can be unacceptably high since typically numerous fusion connections would be required in an optical network. Accordingly, adiabatic coupling cannot be achieved by merely heating the fused region after connection, and is typically not practical in the field.
Some fiber optic transmission systems utilize hybrid spans composed of large effective area, non-dispersion shifted fibers (NDSF) having positive dispersion over the operating window, with compensative fiber, or dispersion compensation fiber (DCF), or dispersion-slope compensation fiber (DSCF), or slope compensating fiber (SCF), having negative dispersion over the operating window. The SCF has a negative dispersion and dispersion slope across the transmission window and will therefore tend to suppress the accumulation of total dispersion and dispersion slope of the NDSF. In general the compensation ratio might be anywhere from one time (and negative) for mirror fibers to five times (and negative) for dispersion compensation fibers. NDSF with effective areas over 100 xcexcm2 have been reported, but the effective areas of the compensative fibers are generally much smallerxe2x80x94on the order of 16-35 xcexcm2xe2x80x94thereby creating a large mode field mismatch.
Since the compensative fiber is utilized as a transmission fiber, rather than as a module, the compensative fiber must be optically coupled to the NDSF with as little splice loss as possible.
In one aspect, a method of connecting optical fibers is disclosed herein. The method comprises providing a first optical fiber having a first refractive index profile, providing a second optical fiber having a second refractive index profile, determining the E-field overlap between the electric field of the first fiber and the electric field of the second fiber at a plurality of times and at one or more wavelengths for the first and second fibers, and selecting a first fusion time corresponding to a desired value of E-field overlap at one or more wavelengths. The method preferably further comprises abutting an end of the first fiber to an end of the second fiber, and heating the abutted ends of the first fiber and the second fiber at the fusion temperature for the first fusion time to form a first juncture.
The first fusion time is preferably selected to correspond to a minimum E-field overlap value for one or more wavelengths.
The first junction preferably comprises a taper having a length of less than about 5 mm.
The heating of the abutted ends of the first fiber and the second fiber preferably comprises a tack splice.
Preferably, the method further comprises providing a third optical fiber, determining the E-field overlap between the electric field of the second fiber and the electric field of the third fiber at a plurality of times and at one or more wavelengths for the second and third fibers, and selecting a second fusion time corresponding to a desired value of E-field overlap at one or more wavelengths. The method further preferably comprises abutting an end of the third fiber to an opposite end of the second fiber from the first junction, and heating the abutted ends of the second fiber and the third fiber at the fusion temperature for the second fusion time to form a second juncture.
In a preferred embodiment, the second fusion time is selected to correspond to a minimum E-field overlap value for one or more wavelengths.
In another embodiment, the first and second fusion times are each selected to yield a minimum splice loss across the first and second junctions for one or more wavelengths.
The second junction preferably comprises a taper having a length of less than about 5 mm.
In another aspect, a method of connecting optical fibers is disclosed herein, the method comprising: providing a first optical fiber having a first refractive index profile; providing a second optical fiber having a second refractive index profile; determining the diffused refractive index profile, at a plurality of times and at one or more wavelengths, of the first fiber being subjected to a fusion temperature; determining the diffused refractive index profile, at a plurality of times and at one or more wavelengths, of the second fiber being subjected to the fusion temperature; determining the electric field of the first fiber at a plurality of times and at one or more wavelengths based upon the diffused refractive index profile of the first fiber as a function of time; determining the electric field of the second fiber at a plurality of times and at one or more wavelengths based upon the diffused refractive index profile of the second fiber as a function of time; determining the E-field overlap between the electric field of the first fiber and the electric field of the second fiber at a plurality of times and at one or more wavelengths for the first and second fibers; and selecting a first fusion time corresponding to a desired value of E-field overlap at one or more wavelengths.
The method preferably further comprises abutting an end of the first fiber to an end of the second fiber, and heating the abutted ends of the first fiber and the second fiber at the fusion temperature for the first fusion time to form a first juncture.
The method further preferably comprises providing a third optical fiber having a third refractive index profile; determining the diffused refractive index profile, at a plurality of times and at one or more wavelengths, of the third fiber being subjected to a fusion temperature; determining the electric field of the third fiber at a plurality of times and at one or more wavelengths based upon the diffused refractive index profile of the third fiber as a function of time; determining the E-field overlap between the electric field of the second fiber and the electric field of the third fiber at a plurality of times and at one or more wavelengths for the second and third fibers; and selecting a second fusion time corresponding to a desired value of E-field overlap at one or more wavelengths.
The method also preferably comprises abutting an end of the third fiber to an opposite end of the second fiber from the first junction and heating the abutted ends of the second fiber and the third fiber at the fusion temperature for the second fusion time to form a second juncture.
In another aspect, an optical waveguide bridge fiber in combination with a first fiber and a second fiber is disclosed herein, the first fiber having a first mode field diameter and positive dispersion in an operating wavelength region between about 1500 nm and about 1650 nm and the second fiber having a second mode field diameter and negative dispersion in the operating wavelength region, wherein one end of the bridge fiber is connected to the first fiber and the other end of the bridge fiber is connected to the second fiber, wherein the bridge fiber comprises a central region extending radially outward from the centerline and having a positive relative refractive index percent, xcex941 %(r) with a maximum relative refractive index percent, xcex941, the central region containing no downdopant, and an inner annular region adjacent and surrounding the central region and having a non-negative relative refractive index percent, xcex942 %(r), with a maximum relative refractive index percent, xcex942, the inner annular region containing no downdopant, and an outer annular cladding region surrounding the inner annular region and having a relative refractive index percent, xcex94c %(r), of essentially 0%.
The bridge fiber preferably has an uncabled fiber cutoff wavelength of less than or equal to about 1500 nm for a 2 m length of bridge fiber.
The length of the bridge fiber is preferably less than or equal to about 50 m.
In a preferred embodiment, the central core region of the bridge fiber preferably has an alpha greater than 1 and less than about 15. In a second preferred embodiment, the central core region has an alpha greater than 1 and less than about 5. In a third preferred embodiment, the central core region has an alpha greater than about 5 and less than about 12.
In a preferred embodiment, the central region has a xcex941 of less than about 1.5%.
In a preferred embodiment, the inner annular region comprises a first annular core segment, adjacent and surrounding the central region and having a non-negative relative refractive index percent.
In a preferred embodiment, the first annular core segment has a positive relative refractive index percent of less than about 0.1%.
In a preferred embodiment, the inner annular region comprises a first annular core segment, adjacent and surrounding the central region and having a non-negative relative refractive index percent, and a second annular core segment, adjacent and surrounding the first annular core segment and having a positive relative refractive index percent.
In another preferred embodiment, the first annular core segment has a relative refractive index percent of essentially 0%.
In a preferred embodiment, the first annular core segment has a positive relative refractive index percent of less than about 0.1%.
In a preferred embodiment, the second annular core segment has a positive relative refractive index percent of less than about 0.4%.
In another aspect, an optical waveguide fiber span is disclosed herein comprising: a first fiber portion comprising a central core segment having a raised relative refractive index profile, a first annular core segment surrounding the central core segment and having a depressed relative refractive index profile, a second annular core segment surrounding the first annular core segment having a raised relative refractive index profile; and a second fiber portion joined to the first fiber portion at a first junction, the second fiber portion comprising a central core segment having a raised relative refractive index profile, a first annular core segment surrounding the central core segment and having a non-negative relative refractive index profile, wherein the E-field overlap at the first junction is between 0.95 and 1.00.
Preferably, the second fiber portion contains no downdopants. Even more preferably, the second fiber portion contains no fluorine.
The second fiber portion preferably further comprises a second annular core segment surrounding the first annular core segment, the second annular core segment having a raised relative refractive index profile and no downdopants.
The E-field overlap at the junction between the first and second fiber portions is preferably between 0.96 and 1.00, even more preferably between 0.98 and 1.00.
Preferably, the first fiber portion is fused to the second fiber portion.
The span further preferably comprises a third fiber portion joined to the second fiber portion at a second junction opposite the first junction, wherein the E-field overlap at the second junction is between 0.95 and 1.00.
In yet another aspect, a method of connecting optical waveguide fibers is disclosed herein, the method comprising: providing a first fiber, providing a second fiber, determining the E-field overlap between the first and second fibers as a function of splicing time, selecting a splicing time corresponding to a desired E-field overlap, and fusing the first fiber to the second fiber for the selected splicing time.
The E-field overlap between the first and second fibers before splicing is preferably greater than about 0.70, more preferably greater than about 0.90.
The E-field overlap between the first and second fibers after splicing is preferably greater than about 0.90, even more preferably greater than about 0.95, and even more preferably greater than about 0.97.
Preferably, the E-field overlap between the first and second fibers before splicing is less than 0.95 and the E-field overlap between the first and second fibers after splicing is greater than about 0.95.
In still another aspect, a method of connecting optical waveguide fibers is disclosed herein, the method comprising: providing a first fiber; providing a second fiber; providing an intermediate fiber having a first end and a second end, wherein the intermediate fiber and the first fiber have a pre-splice E-field overlap of greater than about 0.95, and wherein the intermediate fiber and the second fiber have a pre-splice E-field overlap of greater than about 0.70; fusing the first fiber to the intermediate fiber to form a first junction, wherein the intermediate fiber and the first fiber have a post-splice E-field overlap of greater than about 0.95; and fusing the second fiber to the intermediate fiber to form a second junction, wherein the intermediate fiber and the second fiber have a post-splice E-field overlap of greater than about 0.95.
Preferably, the intermediate fiber and the first fiber have a post-splice E-field overlap of greater than about 0.96.
Preferably, the intermediate fiber and the second fiber have a post-splice E-field overlap of greater than about 0.96, more preferably greater than about 0.97.
Preferably, the splice loss at the first junction is less than about 0.20 dB at 1550 nm, more preferably less than about 0.15 dB at 1550 nm.
Preferably, the splice loss at the second junction is less than about 0.20 dB at 1550 nm, more preferably less than about 0.10 dB at 1550 nm, even more preferably less than about 0.07 dB at 1550 nm.
The overall splice loss for the first and second junctions is preferably less than or equal to about 0.30 dB, more preferably less than or equal to about 0.25 dB, even more preferably less than or equal to about 0.20 dB.
In a preferred embodiment, the intermediate fiber has an uncabled fiber cutoff wavelength of less than or equal to about 1500 nm for a 2 m length of intermediate fiber and the length of the intermediate fiber is less than or equal to about 50 m. In another preferred embodiment, the intermediate fiber has an uncabled fiber cutoff wavelength of less than or equal to about 1500 nm for a length of bridge fiber greater than or equal to about 2 m.
In yet another aspect, a dispersion compensation module is disclosed herein comprising a dispersion compensation fiber, an NDSF fiber, and a bridge fiber having a first end connected to the dispersion compensation fiber and a second end connected to the NDSF, wherein the splice loss attenuation across the dispersion compensation fiber, the bridge fiber and the NDSF is less than about 0.3 dB.
In a preferred embodiment, module has pigtails comprised of the NDSF.
Preferably, none of the bridge fiber is heated after each end has been fusion spliced. Even more preferably, none of the bridge fiber is heated, after each end has been fusion spliced, to such a temperature that causes dopant diffusion in the bridge fiber.
Preferably the majority of the length of the bridge fiber is not tapered, that is, the majority of the length of the bridge fiber preferably has a uniform relative refractive index profile and uniform physical dimensions.
Preferably, the bridge fiber contains no downdopants. Preferably, the bridge fiber contains no fluorine.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Exemplary embodiment of the segmented core refractive index profile of the present invention is shown in each of the figures.